linux-kernel - Re: [PATCH v5 11/21] mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page

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Message-ID: <20201120081123.GC3200@dhcp22.suse.cz>
Date:   Fri, 20 Nov 2020 09:11:23 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     corbet@....net, mike.kravetz@...cle.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        paulmck@...nel.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
        osalvador@...e.de, song.bao.hua@...ilicon.com,
        duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5 11/21] mm/hugetlb: Allocate the vmemmap pages
 associated with each hugetlb page

On Fri 20-11-20 14:43:15, Muchun Song wrote:
[...]
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index eda7e3a0b67c..361c4174e222 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -117,6 +117,8 @@
>  #define RESERVE_VMEMMAP_NR		2U
>  #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
>  #define TAIL_PAGE_REUSE			-1
> +#define GFP_VMEMMAP_PAGE		\
> +	(GFP_KERNEL | __GFP_NOFAIL | __GFP_MEMALLOC)

This is really dangerous! __GFP_MEMALLOC would allow a complete memory
depletion. I am not even sure triggering the OOM killer is a reasonable
behavior. It is just unexpected that shrinking a hugetlb pool can have
destructive side effects. I believe it would be more reasonable to
simply refuse to shrink the pool if we cannot free those pages up. This
sucks as well but it isn't destructive at least.
-- 
Michal Hocko
SUSE Labs